The Distribution of the Mediterranean Snail, Theba Pisana (Mollusca: Helicidae), on Rottnest Island, Western Australia

6 Pages·1991·1.3 MB·

by  Teresa Lebel

THE DISTRIBUTION OF THE MEDITERRANEAN
SNAIL, THEBA PIS AN A (MOLLUSCA : HELICIDAE),
ON ROTTNEST ISLAND, WESTERN AUSTRALIA.
By TERESA LEBEL, Department of Zoology, University of
Western Australia, Nedlands, Western Australia 6009.
ABSTRACT
The spatial distribution of Theba pisarui on Rottnest Island has changed considerably since its
introduction around 1925. Several new colonies have become established and others have
become extinct since the last survey in 1978 by Johnson and Black. In 1978 Theba occupied a
total area of 5.81 km2 which expanded to 7.24 km2 sometime before this survey in which it was
found that the snail occupies only 2.08 km2. The total extent of Theba has declined to 35% of
the area occupied in 1978.
INTRODUCTION
The Mediterranean Snail Theba pisana was introduced on Rottnest Island
in the vicinity of the Thompson Bay settlement between 1925 and 1927.
Surveys in 1936, 1947, 1958 and 1978 (Serventy 1949, Serventy and Storr
1959, Johnson and Black 1978) have investigated the expansion of the
initial colony and the establishment of isolated colonies at the Lighthouse,
Cape Vlamingh, Stark Bay, Conical Hill and The Bluff. This study
documents the distribution of Theba pisarui on Rottnest Island in May and
August 1988 and examines the changes since 1978.
CENSUS METHOD
Distribution was determined by walking several transects (approximately
10'15m apart), within the area of each known colony, with cross transects
to find the boundaries of areas occupied by live and dead animals. Areas
between colonies were censused by several transects. The distance between
these transects varied with the distance between colonies, those closer
together were sampled more intensively (every 5m) than those further
apart (every 20-30m). In this way most of the island was covered.
A complete survey was impossible in the time available in May, so the
survey was completed in a second trip in August. Several boundaries
described during the first part of the survey were checked in the second visit
and found to be little altered (less than 3m change). Most of the island was
surveyed by the author in the two trips, with occasional assistance from
others.
The searches were carried out throughout the day, from early morning to
evening and once or twice at night. Thus some information was obtained
on the variability in activity and observability of Theba during the day and
under various weather conditions which ranged from sunny through to
heavy rain, sometimes within a day.
PRESENT DISTRIBUTION
The present distribution of Theba on Rottnest Island is examined from
west to east. Those colonies which may have originated from the same
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initial colony, as shown in Figure 1 (Serventy and Storr 1959) are discussed
together. Changes in the boundaries since 1978 (Johnson and Black 197q\
as evidenced by dead shells, and present live colonies are compared '
Figure 2. The numbers represent locations mentioned when describing the
present distribution.
The colonies at Cape Vlamingh, Stark Bay and Conical Hill
remain
isolated. The Cape Vlamingh colony has retracted westward and is ,lvnv
now
found extending between Eagle Bay and Radar Hill. The Conical Y\\\\
colony had expanded slightly westward and southward but has since
suffered extinction, leaving only a small extant colony on the southeast fa
of a dune to the south of Conical Hill. The Stark Bay colony has continued
to expand to the southeast, near White Hill, but has died back elsewhere
leaving a remnant on either side of the bitumen road.
I 15*30* f
Figure 1. Map of Rottnest Island showing the progressive colonisation o(Thcba pisana from
1936 to 1958.
The Lighthouse colony had extended slightly in most directions, and
subsequently retracted considerably, leaving several small extant colonies.
The isolated colony near Ricey Beach is presumably a relict of this
expansion and subsequent retraction, as is the colony near the lighthouse
road turnoff. The occupied area now extends to Barkers Swamp in the
north, and along several tracks to the northwest. Only dead shells were
found at the site of the Pink Lake extension of 1978 and also in the area to
the southwest near Lighthouse Swamp. The limit of distribution directly
south of the lighthouse has remained stable.
The colony near Salmon Swamp appears to be a relict of expansion from
the colony west of Gun Hill. The area south of Gun Hill has not been
recolonised since the 1978 survey. The colony at The Bluff remains
isolated, though it has extended to the northwest.
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M3* 30*t
I
113* 30*1
Figure 2. Map of the distribution o fThebu piscina on Rottnest Island showing the boundaries
of distribution of living snails in 1978 and 1988 and the maximal area covered by snails
between the two censuses as judged by the presence of shells. The numbers on the map refer to
locations listed below and mentioned in the text.
1 Cape Vlamingh 12 Salmon Swamp 23 Bathhurst Point
2 Stark Bay 13 Gun Hill 24 Rifle Range Swamp
3 Conical Hill 14 The Bluff 25 Herschell Lake
4 Eagle Bay 15 Rubbish Tip 26 Garden Like
5 Radar Hill 16 Bagdad Like 27 Thompson Bay Settlement
6 White Hill 17 Point Clune 28 Barracks
7 Lighthouse 18 Parakeet Swamp 29 Bickley Swamp
8 Ricey Beach 19 Geordie Bay Settlement 30 Government House Lake
9 Barkers Swamp 20 Padbury’s Flats 31 Porpoise Bay
10 Pink Lake 21 Mt Herschall 32 Henrietta Rocks
11 Lighthouse Swamp 22 Longreach Bay
Between the lakes there has been a retraction to the east, leaving only a
small extant colony around the rubbish tip and fields to the south of Lake
Bagdad. There has been no further expansion at Point Clune, and the
Parakeet Swamp extension has retracted eastward towards the Geordie Bay
Settlement, leaving scattered extant colonies on Padbury Flats and Mt
Herschell.
Colonies in the area from Longreach Bay to south of Bathurst Point have
retracted to a small isolated colony at Rifle Range Swamp and another
larger colony between Herschell and Garden Lakes.
Only a few scattered Thcba were found near the public showers in the
Thompson Bay Settlement, which was thought to extend to the
southeastern shore of the island, incorporating the now isolated Barracks
colony at Bickley Bay. Because of the lack of dead shells, the snails probably
have not been in the intervening area for some years except in the area
immediately around the Barracks. This area was not surveyed in 1978
because only boundaries where expansion could take place were examined
(Black pers comm.) and is therefore not included in the 1978 data listed in
Table 1.
The colony south of Government House Lake has extended westward near
the airstrip but has retracted northward from Porpoise Bay to a narrow band
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extending east up to Bickley Swamp. Between Henrietta Rocks and the
Barracks no shells were found except for a narrow strip next to the
road.
Table 1 summarises the changes in distribution between 1978 and 1988 for
nine colonies of snails. As judged by the presence of dead snails found
beyond the boundaries of the 1978 distribution, all the colonies increased
in area sometime between 1978 and 1988, including two small areas not
found in 1978. In total, the extension beyond 1978 boundaries of
distribution increased the area by 125%. The total area occupied by living
snails in May and August 1988 is only 36% of the area in 1978 and only 29%
of the maximal area covered between 1978 and 1988. Each of the nine
colonies have recently decreased in area so the decline in distributional area
has occurred over the entire island.
Table 1. Areas occupied by colonies ofTheba pisana on Rottnest Island between 1978 and
1988. The map reference numbers refer to locations indicated on Figure 1. and the —
indicates colonies that did not exist in 1978.
Colony Area in km2
Maximum
(map reference number) 1978 1978-1988 1988
Cape Vlamingh (1) 0.22 0.28 0.12
Conical Hill (3) 0.06 0.10 0.001
Stark Bay (2) 0.24 0.35 0.08
Ricey Beach (18) — 0.12 0.06
Lighthouse (7) 2.41 2.93 0.83
Salmon Swamp (12) — 0.02 0.01
The Bluff (14) 0.04 0.07 0.04
East (16-32) 2.84 3.29 0.92
Barracks (28) — 0.08 0.02
Total 5.81 7.24 2.08
DETERMINANTS OF DISTRIBUTION
Johnson and Black (1978) observed that Theba was most common in
stands of Acanthocarpus preissi and associated plant species. The areas of
expansion from the Lighthouse and Stark Bay in 1978 were covered
primarily with these plants.
A similar plant/snail association was observed in this study, with Theba
occurring mainly on Acanthocarpus at Cape Vlamingh and Stark Bay. A
number ofTheba were found in a dense stand of sedge and Acanthocarpus
to the south of Eagle Bay. The colony at Conical Hill was between two
stands of Acacia sp. on the southeastern duneface. Active Theba were
observed on Acacia stems and leaves and also on the ground, on a small
herb. This may be due to the presence of a good ground vegetation cover
and moist conditions between the Acacia stands. Nowhere else was Theba
observed on Acacia, or in mature plantations with much shade and little
ground vegetation, as in the plantations to the west and east of the
lighthouse.
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The boundary of Theba south of the lighthouse, where there is an abrupt
change from Acanthocarpus to Olearia axillaris, has remained stable since
1958 (Johnson and Black 1978). Theba has still not successfully colonised
Olearia stands to the east of the lighthouse, near Bulldozer Swamp, or in the
centre of Point Clune. Several individuals (5-10) were observed in Olearia
stands south of Government House Like, but it is not known whether they
were residents or moving elsewhere.
Colreavy (1977) found that Theba readily ate Olearia leaves, and Black and
Johnson (1978) observed that Olearia bushes at the western edge of the
Conical Hill colony were the preferred resting sites of adults. It seems
therefore, that Theba does not have an aversion to Olearia itself.
During this survey, it was noticed that many of the Olearia bushes around
Cape Vlamingh, and to the east of the lighthouse, have an ‘Ant-honeydew
producing Homopoteran association’. This type of association is fairly
common (Way 1963, CSIRO 1970). The aggressive behaviour of the ants
prevent harmful insects from attacking the plant or Homopoteran.
However the bushes south of Government House Lake where snails were
found do not have this association. Therefore it is possible that the ants are
preventing Theba from successfully colonising Olearia stands at Cape
Vlamingh and east of the Lighthouse.
Johnson and Black (1978) predicted that by 1988, the Stark Bay and
Lighthouse colonies would fuse. The boundaries of these two colonies
have in fact remained separate and relatively unchanged even though the
vegetation in between is the preferred Acanthocarpus. The vegetation is
however, very low (< 15cm), and sparse with sand in between the bushes
and with little undergrowth. There are few high points to which Theba
could climb to aestivate (Boswell, 1974) or escape predators such as the
Bobtail lizard Tiliqua rugosa (Colreavy, 1977). Another reason for the
unchanged boundaries maybe that Theba was and is relatively sparse in this
area.
During the course of this survey, live Theba were observed to be fairly
sparse over most areas of its distribution. Theba densities were greater in the
dips and hollows between dunes, and also on the dune face away from the
prevailing winds (generally the eastern face). This was especially noticeable
in the Conical Hill and Padbury Flats areas, where scattered groups of live
snails could be found only on one face of the dunes.
The activity level of Theba varied with light intensity (more were active in
the late afternoon and at night, or early morning), and moisture (more
snails were observed and were active during or just after a period of
rainfall). Several snails were marked in the area near the lighthouse and
observed every three hours in order to get some idea of how far they would
travel. At night it was found that some snails moved up to 4m in three
hours, while during the day most remained in one place or moved less than
50cm. If it was raining the snails were more active. The boundaries of
distribution of live snails is therefore a little fluid.
Most of the animals observed during the day (not raining), had intact
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epiphragms, hut even the inactive animals observed towards the end of th
day or early in the morning did not. Boswell (1974) observed that Theb^
can enter into and exit from aestivation rapidly, and it would appear th ^
those individuals observed during this survey were utilising this ability on^
daily basis. a
Perry (1978) found that Theba was very sparse, even where it was once
abundant. The Rottnest Island population appears to have suffered
another decline in the period before the 1988 survey, with mostboundarie
retracting from those observed in 1978 by Johnson and Black. As Johnson
and Black (1978) only examined boundaries where expansion could tak
place, their estimate of total area occupied by Theba in 1978 was probabl^
an overestimate. However, as the total area occupied by living snails in Mav
and August 1988 was approximately a third of the area in 1978 and onlv
29% of the maximal area covered between 1978 and 1988, this population
has still suffered a distinct decline in area.
Serventy and Storr (1959) felt that Theba would accomplish the
colonisation of the entire island within a short time as judged by the rapiditv
of the spread. However this hasn’t happened, and not enough is known
about factors affecting the snail to provide an explanation. Fluctuations of
populations have been recorded elsewhere (Krebs 1978), and it would be
interesting to monitor the population of Theba pisana on Rottnest Island
and to determine whether periodic fluctuations occur over long time
periods and perhaps a cause.
ACKNOWLEDGEMENTS
Many thanks to those people who helped with the survey: R. Black, \
Cruse, J. Christianopoulis, C. Dickman, I. McLeod, L. Masini’ P
Speldewinde, D. Williamson, C. Wilson.
REFERENCES
BOSWELL, W. 1974. Aestivation and survival in 3 terrestrial pulmonate
snails. Hons. Thesis, Zoology Dept., University of W.A.
COLREAVY,M. 1977. Factors controlling the distribution ofTheba pisana
(Muller). BSc Hons. Thesis, Univ. W.A.
JOHNSON, M.S., and BLACK, R. 1978. The distribution of Theba pisana
on Rottnest Island. West. Aust. Nat., 14: 140-143.
KREBS, C.J. 1978. Ecology, The experimental analysis of distribution and
abundance. 2nd ed., Harper and Row.
CS1RO, 1970. The Insects of Australia. Melbourne Unviersity Press.
PERRY, D.H. 1978. The Mediterranean snail (Theba pisana) on Rottnest
Island. West. Aust. Nat., 14: 24-25.
SERVENTY, D.L, and STORR, G.M. 1959. The spread of the
Mediterranean snail on Rottnest Island — Part II. West. Aust Nat 6-
193H96. ” *
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